This invention concerns a method and system for recycling volatile organic compounds (VOCs) used in solvent-based paints. The system and method is applicable whenever a closed chamber or booth is used for spray-painting a workpiece, such as a car, to eliminate venting of VOCs to the atmosphere while capturing the VOCs for recycling.
Present day paint spray-booths need to be designed to provide a safe working environment, to prevent pollution of the atmosphere, and to enhance the quality of the product being sprayed. In general, paint spray-booths are equipped with a constant flow of air into and out of the spray-booth to maintain a reduced level of VOCs in the internal atmosphere. This arrangement has led to a number of attempts to reduce the level of VOCs in the exhaust air to minimize environmental pollution. One such attempt involves the use of aqueous-based, rather than solvent-based paints. Such efforts have generally produced inferior paint results and excessively long periods for drying of the workpiece. Limiting the amount of VOCs in solvent-based paints, as reported by at least one manufacturer, does help to reduce VOC emissions. However, substantial VOC emissions still occur when using such paints, and a more comprehensive solution is required.
Another attempt is found in U.S. Pat. No. 4,261,707, which discloses a complex process and system for elimination of paint solvent vapors from an automobile paint spray-booth and curing oven to maintain a healthful and clean environment for workers in the paint booth. The process and apparatus involves vacuum assisted vaporization of solvents from scrubber water for subsequent condensation and recovery of the solvent.
In U.S. Pat. No. 4,444,573, a method was disclosed for using a hydrotropic substance to decrease the level of an organic solvent in an air stream from a paint spray-booth. A hydrotrope is defined as a chemical substance which includes an organic group chemically bonded to a polar group. The function of the hydrotrope is to increase the solubility of a volatile solvent in water. The method involves (a) contacting of solvent laden air from a paint spray-booth with a spray of aqueous hydrotrope, (b) extracting solvent from the aqueous hydrotrope using an organic oil, and (c) stripping the solvent from the oil using a steam stripper followed by condensation and recovery of the solvent. In U.S. Pat. No. 4,620,858, related to the U.S. Pat. No. 4,261,707 patent discussed above, a method and system is disclosed which achieves contact between solvent from paint spray and an organic solvent absorbing liquid, followed by regeneration of the solvent absorbing liquid. This contact is achieved in a chamber separate from the chamber in which the spraying operation is conducted.
In U.S. Pat. No. 4,927,437, a method and apparatus for removing particles, including solvent droplets, from moving air in a paint spray-booth. The apparatus involves a cyclonic separator.
In the instant invention, a closed system is provided such that, in general, air is not vented to the atmosphere during paint spray operation. Rather, a worker in the closed system is provided with an independent source of air, such as grade D fresh air supplied respirators, thus complying with health and safety requirements of the Occupational Safety and Health Administration (OSHA). The use of a supplied air respirator is already mandated by OSHA (29 CFR 1910.134d1). The instant system involves direct condensation of VOCs from the humidified paint booth atmosphere and direct condensation of solvent laden water vapor, rather than vacuum assisted vaporization and subsequent condensation. A controlled temperature and humidity environment for optimal painting conditions is thereby provided while at the same time reducing environmental pollution.
In the method and system of the instant invention, volatile organic compounds (VOCs) liberated in the course of solvent-based painting of a workpiece, are recovered. The atmosphere within a closed spray-booth is controlled at a fixed humidity such that vaporized water, supplied in the form of steam, acts as a carrier for VOCs liberated in the course of spray painting or heat curing of a workpiece. The VOCs may be recovered continuously, including during the spray-painting or heat curing of the workpiece. The VOC laden, humidified air is circulated through a condenser such that VOCs dissolved in the water vapor condense and are directed to a recovery means. The water and solvent are separated, recovered and purified for re-use or placed in an appropriate container for disposal, thereby eliminating the usual practice of expelling VOC laden air into the atmosphere. The system is provided with a flame-free heat source to cure the painted workpiece and to guard against the danger of explosion.
Thus, it will be appreciated by those skilled in the art that the instant disclosure has several objects. One object is to provide a paint spray-booth having a controlled temperature and humidity so that those working in the field will no longer be at the mercy of the elements when it comes to spray-painting a workpiece under optimal conditions of temperature and humidity.
Another object of this invention is to provide a closed system paint spray-booth which substantially reduces emissions of volatile organic compounds into the atmosphere.
Another object of this invention is to provide a paint spray-booth which will enable those working in the field to comply with local, state and federal clean air directives while at the same time providing an efficient, cost-effective and safe working environment for those working in the spray paint industry.
Other objects and advantages of this invention will become apparent from the full disclosure which follows.